Constant envelope path-dependent phase modulation

ABSTRACT

Optical transmitters configured to modulate optical signals with a path-dependent phase modulation scheme. In certain examples, an optical transmitter includes an optical source that emits a carrier waveform, a modulator configured to modulate the carrier waveform according to a path-dependent phase modulation scheme to produce a modulated optical signal, a mapping module configured to map a data payload to the path-dependent phase modulation scheme, each symbol in the path-dependent phase modulation scheme including a concatenation of at least one location bit and a path bit, the at least one location bit identifying an amount of a phase transition in the modulated optical signal and the path bit identifying a direction of the phase transition, and a pulse-shaping filter configured to control the modulator, based on an output from the mapping module, to impose the path-dependent phase modulation scheme on the carrier waveform to generate the modulated optical signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 120 as a continuationof U.S. patent application Ser. No. 16/352,284, filed on Mar. 13, 2019,and titled “CONSTANT ENVELOPE PATH-DEPENDENT PHASE MODULATION” which isincorporated by reference herein in its entirety.

BACKGROUND

Many optical communication systems manipulate light waves to carryinformation. For instance, often a light source (e.g., a laser source)is modulated to change various properties of emitted light, such as anamplitude, phase, or frequency of the light to convey information. Phasemodulation, also called phase shift keying (PSK), is a commonly usedcommunications technique in which information, in symbols, is encodedonto a carrier signal using phase changes. Phase shift modulationprovides log₂(2^(N)) bits per symbol, where N is the number of symbolsin the complex signal plane. For example, binary phase shift keying(BPSK) has two symbols or one bit per symbol. Quadrature phase shiftkeying (QPSK) has four symbols or two bits per symbol.

SUMMARY OF INVENTION

In many applications, it would be desirable to map more bits per symbolwithout introducing additional overhead (e.g., without increasing therequired transmission bandwidth) or latency. Aspects and embodimentsdisclosed herein present such a technique.

According to one embodiment, an optical transmitter is configured totransmit a modulated optical signal and comprises an optical sourceconfigured to emit a carrier waveform, and a modulator configured tomodulate the carrier waveform according to a path-dependent phasemodulation scheme to produce the modulated optical signal. The opticaltransmitter further comprises a mapping module configured to map areceived data payload to the path-dependent phase modulation scheme,each symbol in the path-dependent phase modulation scheme including aconcatenation of at least one location bit and at least one path bit,the at least one location bit corresponding to a current location of thesymbol in a symbol constellation corresponding to the path-dependentphase modulation scheme, and the at least one path bit representing aclockwise or counter-clockwise rotation from a prior location of thesymbol to the current location of the symbol in a complex plane of thesymbol constellation, and a pulse-shaping filter configured to controlthe modulator, based on an output from the mapping module, to impose thepath-dependent phase modulation scheme on the carrier waveform togenerate the modulated optical signal.

In one example, the at least one location bit represents a value oflower order bits at the current location of the symbol.

In another example, the at least one path bit is a highest order bit inthe symbol.

In one example, the clockwise or counter-clockwise rotation is arotation of 180 degrees.

In another example, the clockwise rotation represents a zero bit and thecounter-clockwise rotation represents a one bit.

The optical transmitter may further comprise a forward error correctionmodule configured to receive the data payload and to apply a forwarderror correcting code to the data payload.

In one example, the modulator is an electro-optic modulator.

According to another embodiment, an optical transmitter configured totransmit a modulated optical signal comprises an optical sourceconfigured to emit a carrier waveform, a modulator configured tomodulate the carrier waveform according to a path-dependent phasemodulation scheme to produce the modulated optical signal, a mappingmodule configured to map a received data payload to symbols according tothe path-dependent phase modulation scheme, each symbol, S, including aconcatenation of at least one location bit, L, and at least one pathbit, P, such that S=[PL], where L represents log₂(2^(N)) bits, therebeing N symbol locations defined in a symbol constellation correspondingto the path-dependent phase modulation scheme, and a pulse-shapingfilter configured to control the modulator, based on an output from themapping module, to impose the path-dependent phase modulation scheme onthe carrier waveform to generate the modulated optical signal, eachsymbol corresponding to a phase transition in the modulated opticalsignal, with the at least one location bit, L, identifying an amount ofphase change corresponding to the phase transition, and the at least onepath bit, P, identifying whether the phase change is positive ornegative.

In one example, the amount of phase change corresponding to the phasetransition is pi radians.

The optical transmitter may further comprise a forward error correctionmodule configured to receive the data payload and to apply a forwarderror correcting code to the data payload.

In one example, a positive phase change represents a one bit and anegative phase change represents a zero bit.

According to another embodiment, an optical transmitter configured totransmit a modulated optical signal comprises an optical sourceconfigured to emit a carrier waveform, a modulator configured tomodulate the carrier waveform according to a path-dependent phasemodulation scheme to produce the modulated optical signal, a mappingmodule configured to map a received data payload to the path-dependentphase modulation scheme, each symbol in the path-dependent phasemodulation scheme including a concatenation of at least one location bitand at least one path bit, the at least one location bit identifying anamount of a phase transition in the modulated optical signal and the atleast one path bit identifying a direction of the phase transition, anda pulse-shaping filter configured to control the modulator, based on anoutput from the mapping module, to impose the path-dependent phasemodulation scheme on the carrier waveform to generate the modulatedoptical signal.

In one example, the amount of the phase transition is an integermultiple of pi radians.

In another example, the at least one path bit is a highest order bit inthe symbol.

In another example, each symbol, S, is defined by S=[PL], where Lcorresponds to the at least one location bit and represents log₂(2^(N))bits, there being N symbol locations defined in a symbol constellationcorresponding to the path-dependent phase modulation scheme, and Pcorresponds to the at least one path bit.

In one example, a clockwise direction of the phase transitioncorresponds to a zero bit and a counter-clockwise direction of the phasetransition corresponds to a one bit.

The optical transmitter may further comprise a forward error correctionmodule configured to receive the data payload and to apply a forwarderror correcting code to the data payload.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment. Variousaspects and embodiments described herein may include means forperforming any of the described methods or functions.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the disclosure. In thefigures, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in every figure.In the figures:

FIG. 1 is a diagram of the complex plane representation of a binaryphase shift keying (BPSK) signal constellation;

FIG. 2 is a functional block diagram of one example of a receiveraccording to aspects of the present invention;

FIG. 3 is a diagram illustrating an example of operation of an etalonaccording to aspects of the present invention;

FIG. 4 is a diagram illustrating an example of an enhancedpath-dependent BPSK modulation scheme according to aspects of thepresent invention;

FIG. 5 is a diagram illustrating an example of an enhancedpath-dependent phase modulation scheme according to aspects of thepresent invention; and

FIG. 6 is a functional block diagram of one example of a transmitteraccording to aspects of the present invention.

DETAILED DESCRIPTION

Aspects and embodiments are directed to methods and apparatus fordetecting the direction of phase change in a phase-modulated signal, andusing that information to encode additional information onto thephase-modulated signal by increasing the number of bits per symbol. Asdiscussed in detail below, this additional information can be added tothe signal without increasing the required bandwidth for the signal andwithout introducing undesirable effects into the signal, such as latencyerrors or changes in the signal envelope (amplitude variations).

As discussed above, phase shift modulation provides log₂(2^(N)) bits persymbol, where N is the number of symbols in the complex signal plane.The value of each symbol is determined by the location of the symbol inthe plane. For example, FIG. 1 is the complex plane representation of abinary phase shift keying (BPSK) signal constellation. In the diagram,the vertical axis 102 is the imaginary axis and the horizontal axis 104is the real axis. There are two symbols 112, 114 in the plane, both ofwhich are located on the horizontal axis 104. The first symbol 112located to the right of the origin is at a phase angle of 0 radians,while the second symbol 114 to the left of the origin is located at aphase angle of pi radians (180 degrees). The phase angle ϕ is determinedby the rotation of a vector originating at the origin and pointing alongthe horizontal axes in the positive (towards the right in FIG. 1)direction. Such a vector rotating in the counter-clockwise directionconstitutes a positive phase rotation, indicated by arrow 122. In theexample shown in FIG. 1, a rotation of pi radians is shown. A vectorrotating in a clockwise direction constitutes a negative rotation,indicated by arrow 124; in this case a rotation of −pi radians.

In conventional BPSK communication, a receiver is unable to determinewhether a transmit modulator rotated the signal phase in a clockwise orcounter-clockwise direction to reach the next phase state. In otherwords, in conventional BPSK communication, the receiver cannot tell thedifference between a rotation of +pi and a rotation of −pi. Aspects andembodiments provide a component of the receiver (e.g., the demodulator)with the ability to make such a determination. Accordingly, furtheraspects and embodiments are directed to a modulation scheme thatexploits the ability to determine the direction of phase change whenmoving between phase states, in order to provide an additionalinformation bit. While the approach is explained below using BPSK, thismethod is applicable to all M-ary (e.g., binary, quaternary, etc.) andQAM modulation schemes.

FIG. 2 is a block diagram of one example of an optical receiveraccording to certain embodiments. The optical receiver 200 receives amodulated optical signal 210 transmitted along a free space signal path(e.g., free space optical, FSO), a fiber coupling, or another waveguidesystem from a transmitter (not shown). The optical receiver 200 includesa phase change detector 220 that includes an optical resonator assembly225, a detector assembly 230 including at least one optical-electricalconverter (OEC) 235, and a signal processing circuit 240. In certainexamples, the detector assembly 230 and the signal processing circuit240 may be collectively referred to as a detector. The detector assembly230 and the signal processing circuit 240 may be separate components ormay be part of a common module. The optical resonator assembly 225 ispositioned to receive the modulated optical signal 210 and to produce anoutput optical signal 215 that has characteristics representative of themodulation of the modulated optical signal 210, as discussed furtherbelow. The detector assembly 230 receives the output optical signal 215from the optical resonator assembly 225 and the at least one OEC 235converts the optical signal into an electrical signal that can beprocessed by the signal processing circuit 240 to produce a decodedinformation signal 245. The decoded information signal 245 may includethe information that was encoded on the modulated optical signal 210 bythe modulation of the modulated optical signal 210. The OEC 235 mayinclude one or more photo-diodes, for example, or other componentscapable of transforming an optical signal into an electrical signal. Thesignal processing circuit 240 may include various components, as will beunderstood by those skilled in the art, such as analog-to-digitalconverters, filters, amplifiers, controllers, etc., to condition andprocess the electrical signals received from the detector assembly 230to produce the decoded information signal 245.

In certain examples, the optical resonator assembly 225 includes one ormore optical resonators configured to convert phase modulation of themodulated optical signal 210 into an intensity modulation, containingboth phase change and direction of phase change, of the output opticalsignal 215. As used herein, the term “optical resonator” refers to acomponent capable of sensing variations, such as frequency variations,amplitude variations, or phase variations in the received optical signal210. Examples of optical resonators may include Fabry-Perot etalons,micro-rings, or other types of optical resonators. Each opticalresonator in the optical resonator assembly 225 converts the modulationof the arriving optical signal 210 in part by interaction of thearriving optical signal 210 with optical energy built-up in theresonator. Individual optical resonators within the optical resonatorassembly 225 may be operated in a resonant or non-resonant mode.Operation of an optical resonator as a phase change detector isdiscussed below using the example of an etalon; however, those skilledin the art will appreciate that other types of optical resonators can beoperated according to similar principles. With the inclusion of one ormore optical resonators, the optical resonator assembly 225 provides aphase change detector that can also detect the direction of the phasechange, as discussed in more detail below.

Referring to FIG. 3, in certain examples an etalon 300 is a componenthaving a pair of parallel semi-reflective surfaces 312, 314 that mayinclude a dielectric material in between, and has a characteristicresonant frequency associated with a certain wavelength of light basedupon the spacing (i.e., dimension 302) between the semi-reflectivesurfaces. The surfaces 312, 314 are semi-reflective and alsosemi-transmissive, in that they allow some light through, and thereforethe arriving modulated optical signal 210 may be allowed into the etalon300 and may resonate inside the etalon (i.e., in the interior 316between the two semi-reflective surfaces 312, 314). Additionally, someof the light resonating inside is allowed out of the etalon 300 (throughat least one of the semi-transmissive surfaces). Light emerging from theetalon 300 is shown, for example, as the output optical signal 215.

The optical signal 210 received by the etalon 300 establishes asteady-state energy-conserving condition in which optical signal energycontinuously arrives at the etalon 300, adds to the built-up resonatingenergy existing inside the etalon 300, and emerges from the etalon 300at a constant rate. If the frequency, amplitude, or phase of thearriving optical signal 210 changes, this change causes a temporarydisruption to the resonance inside the etalon 300 and the lightintensity emerging from the etalon 300 is also disrupted, until a steadystate condition is re-established. Accordingly, a change in phase,frequency, or amplitude of the arriving optical signal 210 causes achange in intensity of the output optical signal 215. Thus, the etalonfunctions as a modulation converter, for the optical signal 210. Theoutput optical signal 215 may therefore carry the same informationcontent as the arriving optical signal 210, but in an intensitymodulated form, rather than a phase modulated form, for example.

FIG. 3 illustrates an example of the above-described operation of theetalon 300. FIG. 3 shows a graph 320 of the arriving modulated opticalsignal 210, showing a phase change in the optical signal 210. The graph320 plots the phase (vertical axis) of the optical signal 210 over time(horizontal axis), showing a phase transition of pi (180 degrees) atpoint 322. FIG. 3 also shows a graph 330 of optical signal intensity (asoutput power) emerging from the etalon 300 during the phase transitionin the received optical signal 210. At region 332 the etalon 300 is in asteady-state resonance condition wherein a steady intensity of lightemerges. At point 334, corresponding to point 322 in the graph 320, aphase transition occurs in the arriving optical signal 210, temporarilydisrupting the steady-state and causing a drop in the emerging lightintensity. During successive reflections inside the etalon 300, andindicated region 336 in the graph 330, resonance is re-establishing, andthe emerging light intensity increases until, at point 338, a steadyintensity of light emerges when the etalon 300 has returned to asteady-state condition. Thus, variations in the intensity of the outputoptical signal 215 from the etalon 300 indicate that a transitionoccurred in the arriving optical signal 210, such as a phase transitiondue to phase modulation of the optical signal 210.

The etalon 300 may have varying levels of reflectivity of thesemi-reflective surfaces 312, 314. In certain examples, the reflectivitymay be expressed as a fraction of light amplitude reflected back intothe interior 316, or may be expressed as a fraction of light intensityreflected back into the interior 316. The reflectivity of each of thefirst and second semi-reflective surfaces 312, 314 may be the same ordifferent, and may be any suitable value for a particularimplementation. The etalon 300 is one example of a suitable opticalresonator in accord with aspects and embodiments described herein.However, the use of the term “etalon” throughout this disclosure is notintended to be limiting and as used herein may include any of multiplestructures, including plates with reflecting surfaces as well asparallel mirrors with various materials in between, and may also bereferred to as cavities, interferometers, and the like. Additionally,etalon structures may be formed as a laminate, layer, film, coating, orthe like. In some examples, an etalon may include reflective surfaces(including semi-reflective surfaces) that are not co-planar and/or arenot co-linear. For example, an interior reflective surface of an etalonmay include some curvature, and an opposing surface may also be curvedsuch that a distance between the two surfaces is substantially constantacross various regions of the etalon, in some examples. In otherexamples, an etalon may have non-linear or non-planar surfaces withvarying distances between the surfaces at various regions and may stillfunction as an optical resonator for various wavelengths and at variousregions, suitable for use in examples discussed herein. Accordingly, anetalon may be purposefully designed to conform to a surface, or to havevarious regions responsive to differing wavelengths, or responsive todiffering angles of arrival for a given wavelength, in certain examples.Additionally, other optical resonators, such as an optical loop ormicro-ring, for example, may operate according to similar principles andalso be used in the optical resonator assembly 225. For example, opticalsignal energy accumulated in the loop/micro-ring may constructively ordestructively interfere with itself, at certain frequencies(wavelengths), and such constructive or destructive interaction may bedisturbed by a phase change in an arriving optical signal 210.Accordingly, phase changes in the arriving optical signal 210 may bedetected and interpreted to demodulate the arriving optical signal 210.

Any change in amplitude or phase of the arriving optical signal 210 atthe optical resonator assembly 225 may cause a variation in the outputoptical signal 215. In the example discussed above, the etalon 300 isoperated in a resonant mode or condition. However, wavelengths of thearriving optical signal 210 that do not create a resonant response inthe etalon 300 may nonetheless establish the output optical signal 215.Under such a condition, the etalon 300 (or other optical resonator) maybe described as being untuned or detuned from the particularwavelength(s). A phase transition in the modulated optical signal 210received by a detuned optical resonator causes a disturbance in theoutput optical signal from that resonator despite the lack of resonance.In addition, a phase shift in one direction may cause a greaterdisturbance in a particular resonator than a phase shift of the samemagnitude in an opposite direction (e.g., a positive phase shift versusa negative phase shift may cause different disturbances in the outputoptical signal from an optical resonator).

According to certain examples, an optical resonator, such as the etalon300, will develop a steady-state output signal based on the inputsignal, and maintain a given level of the output optical signal 215until a phase transition of the arriving optical signal 210 occurs. Asdiscussed above, when a phase modulation transition occurs in the inputoptical signal 210, self-interference (constructive or destructive) maycause a phase-dependent transient disturbance in the intensity of theoutput optical signal 215. Such a transient disturbance may depend uponthe tuning of the etalon (or other optical resonator). Accordingly, theoptical resonator assembly 225 may include one or more etalons 300 (orother optical resonators) having various states of tuning relative to awavelength, λ of the arriving optical signal 210. For example, referringagain to FIG. 3, a tuned etalon 300 may have an optical interiordimension 318 (e.g., based upon the speed of light in the material ofthe interior 316) that is an integer number of half-wavelengths, e.g.,L=nλ/2. A detuned etalon may be positively detuned by having a slightlylarger dimension, e.g., L=nλ/2+ε, or be negatively detuned by having aslightly smaller dimension, e.g., L=nλ/2−ε. In some embodiments, thedimensional variant, c, may have a nominal value of one eighthwavelength, e.g., ε=λ/8. In other embodiments, the dimensional variantmay have a nominal value of a tenth of a wavelength, e.g., c=λ/10, or atwelfth of a wavelength, ε=λ/12. Other embodiments may have differentnominal dimensional variants, ε, and any dimensional variant, ε, may bemore or less precise in various embodiments. Additionally, a positivelydetuned optical resonator with respect to a particular wavelength may bea negatively detuned optical resonator with respect to anotherwavelength. Accordingly, in some embodiments, tuning relative to aparticular wavelength may be less significant than a difference intuning between two or more optical resonators. For example, thepositively and negatively detuned resonator dimensions discussed abovemay be equivalently described with respect to two optical resonators asbeing detuned by 2ε relative to each other, without regard to whatwavelength might produce resonance in either of the optical resonatorsand/or an alternate or additional tuned resonator.

As shown in FIG. 3 and discussed above, the output intensity/power froma tuned etalon 300 exhibits a transient disturbance (a temporaryreduction in power in the example shown in FIG. 3 although in otherconfigurations the transient disturbance may instead be a temporaryincrease) in response to a phase transition occurring in the arrivingmodulated optical signal 210. Similarly, a phase transition in thearriving modulated optical signal 210 causes a transient disturbance inthe output intensity from a detuned etalon; however, the transientdisturbance is different depending on whether the phase transition is aphase advance (e.g., +pi) or a phase retreat (e.g., −pi). For example, apositively detuned etalon may have a more pronounced or prominent (e.g.,larger transient peak or dip) response to a phase advance than to aphase retreat of the same value. In other words, the change in intensityof the output signal 215 from a positively detuned etalon may be greaterin response to a phase advance (e.g., +pi) than to a phase retreat(e.g., −pi). Similarly, the change in intensity of the output signal 215from a negatively detuned etalon may be greater in response to a phaseretreat (e.g., −pi) than to a phase advance of the same value (e.g.,+pi). These differences in the response from the etalon based on itstuning state may allow the receiver to distinguish between positive andnegative phase transitions in the arriving modulated optical signal 210.

The response and operation of various examples of etalons and otheroptical resonators in different tuning states to different phase (andother modulation) transitions in an arriving optical signal arediscussed in more detail in commonly-owned U.S. Patent Publication No.2018-0145764 titled “DEMODULATION OF QAM MODULATED OPTICAL BEAM USINGFABRY-PEROT ETALONS AND MICRORING DEMODULATORS” and U.S. PatentPublication No. 2018-0102853 titled “SYSTEMS AND METHODS FORDEMODULATION OF PSK MODULATED OPTICAL SIGNALS,” each of which is hereinincorporated by reference in its entirety for all purposes.

In certain examples, the optical resonator assembly 225 may includemultiple optical resonators with different tuning or detuning states. Asdiscussed above, a general phase transition of any size and directionmay be detected and distinguished by a combination of any two opticalresonators, at least one of which is detuned from the wavelength of thearriving optical signal. In a first example, a tuned optical resonatorexhibits a temporary reduction in output signal intensity and the amountof phase transition may be determined by the amount of reduction inoutput signal intensity. The direction of the phase transition (advanceor retreat) may be determined by analyzing the output signal intensityfrom a detuned optical resonator, e.g., for a positively detunedresonator, a positive peak indicates a positive phase transition and nopositive peak (or a minor negative peak) indicates a negative phasetransition. In a second example, a positively detuned optical resonatordetects positive phase transitions by producing a positive output peakwhose amplitude is indicative of the amount of the phase transition, anda negatively detuned optical resonator detects negative phasetransitions by producing a positive output peak whose amplitude isindicative of the amount of the phase transition. Accordingly, either ofthe first example or the second example (each having a pair of opticalresonators) may uniquely distinguish both phase advances and phaseretreats and the amount of the phase advance or retreat.

Additionally, analysis of the output optical signal from two or moreoptical resonators may identify whether either is tuned (resonant)(e.g., relatively high output intensity, similar response to bothpositive and negative phase transitions) and/or detuned. Further, areceived optical signal that drifts in wavelength may cause a tunedoptical resonator to become a detuned optical resonator, and vice-versa.Accordingly, in certain examples, the signal processing circuit 240 maybe configured (e.g., programmed) to analyze output intensities from twoor more optical resonators to determine whether they are detuned orwhether one is tuned, and interpret the output signals accordingly.While distinction of any general phase transition is achievable bysystems and methods described herein having only two optical resonators,various embodiments may operate more robustly having three or moreresonators, for example, such that there is no possibility of having torely on two optical resonators, e.g., that may each be detuned in thesame direction relative to an arriving optical signal wavelength.Accordingly, to account for an arriving optical signal having a generalwavelength, or to account for an arriving optical signal subject towavelength drift or variation, or to account for dimensional changes ofthe optical resonators, e.g., due to temperature, manufacturingtolerance, or the like, in certain embodiments the optical resonatorassembly 225 may include three or more optical resonators each having anoptical dimension different from the others by some amount, such as butnot limited to a fixed dimensional variant, ε, as discussed above. Inaddition, with knowledge of the wavelength of the arriving modulatedoptical signal 210, such that a single optical resonator can beappropriately detuned, and calibration of the receiver to the responsefrom the optical resonator, a single detuned optical resonator can beused to detect and distinguish a positive phase transition (e.g., +pi)from a negative phase transition (e.g., −pi).

As discussed above with reference to FIG. 2, the intensity-modulatedoutput optical signal(s) 215 from one or more optical resonators (e.g.one or more etalons 300) in the optical resonator assembly 225 may beconverted to an electrical signal by an optical-electrical converter(OEC) 235, which may include a photodetector, such as a photodiode, forexample. The output of the OEC 235 may be an amplitude-modulated versionof the intensity-modulated optical signal 215, which may be provided tothe signal processing circuit 240 for processing. The signal processingcircuit 240, which may include components such as an analog to digitalconverter, correlator, code generator, various amplifiers, filters, orother analog or digital components as would be recognized andappreciated by those skilled in the art, processes the signal receivedfrom the OEC 235 to retrieve the information-carrying content of theoptical signal 210. Thus, the optical resonator assembly 225 is a phasechange detector that can also detect the direction of the phase change.For example, referring again to FIG. 1, in BPSK the optical resonatorassembly 225 provides the ability to distinguish whether the phasevector rotated in the positive pi direction (arrow 122) or the negativepi direction (arrow 124). This distinction provides an additional bit ofinformation. Thus, in BPSK, for example, two bits of information can beencoded per symbol, instead of only one bit per symbol, without therebeing any bandwidth or latency penalties. Accordingly, aspects andembodiments are directed to a path-dependent phase modulation schemethat leverages with the ability of the optical receiver 200 to not onlydetect a phase change in the arriving optical signal 210, but also todetermine the direction of that phase change (e.g., +pi or −pi for aBPSK signal), to encode additional information in a phase-modulatedoptical signal. Advantageously, because the direction of a phasetransition can be determined, embodiments of the path-dependent phasemodulation scheme presented herein encode the additional informationwithout adding phase transitions, essentially achieving double the bitrate for the same signal bandwidth. In addition, the approach disclosedherein maintains a constant signal envelope, thereby maintaining theadvantages by phase modulation (e.g., over amplitude modulation) withrespect to channel noise.

FIG. 4 is a diagram illustrating an example of a path-dependentmodulation concept according to certain aspects and embodiments. Thisexample is explained using BPSK; however, those skilled in the art willappreciate, with the benefit of this disclosure, that the approach canbe applied similarly to all M-ary and QAM modulation schemes. Accordingto certain embodiments, an additional bit is assigned based on thedirection of rotation to the next phase state. In the examples shown inFIG. 4, a clockwise rotation has been arbitrarily chosen to represent azero bit (shown in dashed lines) and a counter-clockwise rotation torepresent a one bit (shown in dotted lines). The bit represented by therotation is referred to herein as the path bit, P. The bit representedby the symbol's location is referred to herein as the location bit, L. Acomplete symbol, S, is formed by concatenating the location bit with thepath bit, S=[PL], where L is the lower order bit. Table 1 is a statetransition table for an example of this modulation scheme.

TABLE 1 Symbol Symbol Value [PL] Location Value 00 01 10 11 0 −2π  −π+2π  +π 1  −π −2π  +π +2π

In Table 1, the first column represents the current symbol location bitvalue. A zero in the first column represents location bit zero at aphase angle of pi radians (114). A one in the first column represents alocation bit of one at a phase angle of zero radians (112). Thehorizontal header [00 01 10 11] represents the symbol value as afunction of destination phase state and path to the destination state.The table values represent the direction and degree of rotation inradians that are used to generate a particular symbol value. Table 1 isconsistent with the transitions shown in FIG. 4. According to oneembodiment, Table 1 can be used as follows: Assume the current symbollocation value is zero and the next two bits to be transmitted are 11.The intersection of row zero and column [11] is +pi. This means that thesystem must advance to the symbol location one state by changing thephase by +pi radians. It should be noted that when the current state andthe next state are the same, a 2pi rotation is not needed.

As noted above, in the example shown in FIG. 4 and presented in Table 1,a clockwise rotation has been arbitrarily chosen to represent a zero bitand a counter-clockwise rotation to represent a one bit. It will bereadily apparent to those skilled in the art, given the benefit of thisdisclosure, that Table 1 can be modified to accommodate the oppositecase in which a counter-clockwise rotation represents a zero bit and aclockwise rotation represents a one bit. The principles of themodulation scheme disclosed herein apply equally to either scenario, andembodiments are not limited to any one specific arrangement. Further, inthe example discussed above, the location bit is the lower order bit;however, in other examples, the location bit may be the higher orderbit.

According to certain examples, the following approach can be implementedto apply the path data bit to a standard M-ary or QAM constellation. ForM-ary and QAM modulation schemes, there are N symbol locations definedin the symbol constellation. Each location L represents log₂(2^(N))bits. In one example, L_(n) represents the value of the lower order bitsat symbol location n, where n=1, 2, . . . , N. A path P is defined as aclockwise or counter-clockwise rotation from one symbol location toanother in the complex plane. In one example a path that is acounter-clockwise rotation about the complex plane origin constitutes apositive phase rotation, and path that is a clockwise rotation about thecomplex plane origin constitutes a negative phase rotation. However, asnoted above, in other examples the opposite arrangement can be used. Fortwo paths, of opposite rotation, originating from a particular locationL₁ in the complex plane and terminating at the location L₂ in thecomplex plane, the path values are complemented. L₁ and L₂ may be thesame or different locations. If L₁ and L₂ are the same location, therotation will be +2pi or −2pi. In one example, P determines the highestorder symbol bit. The symbol value is determined by concatenating thesymbols location value and the path taken to get to a particular symbol.Thus, S_(n) represents the symbol value of a symbol at position n, withS_(n)=[PL.]. As discussed above, in other examples, L_(n) can representthe value of higher order bits, such that S_(n)=[L_(n)P].

Table 2 below presents an example of phase rotations used to implementthe above-discussed modulation scheme. Table 2 is consistent with thetransitions shown in FIG. 5.

TABLE 2 Next State Current State 00 01 10 11 00 0 +π −π −2π  01 −π 0−2π  +π 10 +π +2π  0 −π 11 +2π  −π +π 0

As discussed above, examples of the path-dependent modulation schemedisclosed herein advantageously includes the additional bits ofinformation (the path bit) without increasing the bandwidth of themodulated optical signal and without requiring any changes in theamplitude of the optical signal. That is, the modulation schemedisclosed herein allows for maintaining a constant envelope signal. Thismay be particularly advantageous and/or desirable because the constantenvelope or signal amplitude is a feature that makes phase modulation(PSK) preferable over intensity or amplitude modulation for manyapplications. Although some conventional modulation techniques haveattempted to include an additional bit of information per symbol, thesehave required the introduction of variations in the signal envelope,which is contrary to the methods typically used in phase modulation. Thepath-dependent modulation scheme and approaches disclosed hereinmaintain a constant envelope, thereby maintaining the advantages offeredby phase modulation with respect to channel noise. Further, conventionalhigher-order modulation schemes that provide two bits per symbol, havepenalties in bandwidth or latency, that are both avoided withembodiments of the path-dependent phase modulation approach disclosedherein.

An optical transmitter can be configured to implement examples of theabove-discussed modulation scheme, using both path and location bits, togenerate the modulated optical signal 210 that may be received anddemodulated by embodiments of the receiver 200, for example. FIG. 6 is afunctional block diagram of one example of an optical transmitter 400according to certain embodiments. The transmitter 400 illustrated inFIG. 6 may be combined with the receiver 200 illustrated in FIG. 2 toprovide one example of a communication assembly, as will be readilyapparent to one of ordinary skill in the art given the benefit of thisdisclosure.

Though the components of the example optical transmitter 400 shown inFIG. 1 and the optical receiver 200 shown in FIG. 2 may be shown anddescribed as discrete elements in a block diagram, and may be referredto as “module”, “circuitry”, or “circuit,” unless otherwise indicated,the components may be implemented as one of, or a combination of, analogcircuitry, digital circuitry, or one or more microprocessors executingsoftware instructions (e.g., predefined routines). In particular, thesoftware instructions may include digital signal processing (DSP)instructions. Unless otherwise indicated, signal lines betweencomponents of the optical transmitter 400 and components of the opticalreceiver 200 may be implemented as discrete analog, digital, or opticalsignal lines. Some of the processing operations may be expressed interms of calculations or determinations by the optical transmitter 400,the optical receiver 200, a controller, or other components. Theequivalent of calculating and determining values, or other elements, canbe performed by any suitable analog or digital signal processingtechniques and are included within the scope of this disclosure. Unlessotherwise indicated, control signals may be encoded in either digital oranalog form.

Referring to FIG. 6, one example of an optical transmitter 400 mayinclude an input to receive a data payload 410, a forward errorcorrection (FEC) module 420, a mapping module 430, a pulse-shapingfilter 440, an optical source (e.g., a laser) 450, a modulator 460, andoptics 470 and an output to provide the modulated optical signal 210.

In the transmitter 400, the FEC module 420 implements forward errorcorrection by adding redundancy to the data with a forward errorcorrecting code, such as a block code or convolution code. For example,the FEC module 420 may repeat one or more bits within the data payload410 to reduce an effect that the transmission medium may have on thetransmitted signal waveform. Accordingly, in various examples theoptical transmitter 400 may include the FEC module 420 to control errorsthat may result from transmitting the data payload 410 through a noisyor lossy medium.

The mapping module 430 maps the data payload to a particular modulationscheme, such as various positions of a particular phase and amplitudeconstellation. The mapping module 430 may implemented examples of thepath-dependent modulation scheme discussed above with reference to FIGS.4 and 5 and Tables 1 and 2. A pulse-shaping filter 440 may receiveoutput of the mapping module 430 and control the modulator 460 to imposethe modulation scheme on the optical source 450 to generate themodulated optical signal 210. In various examples, the modulator 460 maybe an electro-optic modulator, and may include the optical source 450,such as a laser. In particular, the optical source 450 may emit acontinuous carrier waveform that is modulated in phase for each symbolof the data payload to encode those symbols on the carrier waveform. Thetransmitter 400 may also include various optics 470 such as one or moremirrors or lenses to output the modulated optical signal 210.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention. Itis to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in theforegoing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.Any references to front and back, left and right, top and bottom, upperand lower, and vertical and horizontal are intended for convenience ofdescription, not to limit the present systems and methods or theircomponents to any one positional or spatial orientation. The termslight, light signal, and optical signal may be used interchangeablyherein and refer generally to an electromagnetic signal that propagatesthrough a given medium, which may be empty space, e.g., a vacuum, or maybe an atmospheric, e.g., air, or other medium, such as fiber or otheroptics components. The terms “light,” “light signal,” and “opticalsignal” are not meant to imply any particular characteristic of thelight, such as frequency or wavelength, band, coherency, spectraldensity, quality factor, etc., and may include radio waves, microwaves,infrared, visible, and/or ultraviolet electromagnetic radiation, orother non-ionizing electromagnetic radiation conventionally processed inthe field of optics.

Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. A communications apparatus configured to transmita modulated signal, comprising: a source configured to emit a carrierwaveform; a modulator configured to modulate the carrier waveformaccording to a path-dependent phase modulation scheme to produce themodulated signal; and a mapping module configured to map a received datapayload to the path-dependent phase modulation scheme, each symbol S inthe path-dependent phase modulation scheme including a concatenation ofat least one location bit L and at least one path bit P.
 2. Thecommunications apparatus of claim 1, wherein the at least one locationbit corresponds to a current location of the symbol in a symbolconstellation corresponding to the path-dependent phase modulationscheme, and the at least one path bit indicates a clockwise orcounter-clockwise rotation from a prior location of the symbol to thecurrent location of the symbol in a complex plane of the symbolconstellation.
 3. The communications apparatus of claim 2, furthercomprising: a pulse-shaping filter configured to control the modulator,based on an output from the mapping module, to impose the path-dependentphase modulation scheme on the carrier waveform to generate themodulated signal; and a forward error correction module configured toreceive the data payload and to apply a forward error correcting code tothe data payload.
 4. The communications apparatus of claim 2, whereinthe at least one location bit represents a value of lower order bits atthe current location of the symbol.
 5. The communications apparatus ofclaim 2, wherein the clockwise or counter-clockwise rotation is arotation of 180 degrees.
 6. The communications apparatus of claim 2,wherein the clockwise rotation represents a zero bit and thecounter-clockwise rotation represents a one bit.
 7. The communicationsapparatus of claim 1, wherein the at least one path bit is a highestorder bit in the symbol.
 8. The communications apparatus of claim 1,wherein the communications apparatus is an optical transmitter, themodulator is an electro-optic modulator, the source is an optical sourceand the modulated signal is a modulated optical signal.
 9. Thecommunications apparatus of claim 1, wherein S=[PL], where L representslog₂(2^(N)) bits, there being N symbol locations defined in a symbolconstellation corresponding to the path-dependent phase modulationscheme.
 10. The communications apparatus of claim 9, further comprising:a pulse-shaping filter configured to control the modulator, based on anoutput from the mapping module, to impose the path-dependent phasemodulation scheme on the carrier waveform to generate the modulatedsignal, each symbol corresponding to a phase transition in the modulatedsignal, with the at least one location bit, L, identifying an amount ofphase change corresponding to the phase transition, and the at least onepath bit, P, identifying whether the phase change is positive ornegative.
 11. The communications apparatus of claim 10, wherein theamount of phase change corresponding to the phase transition is piradians.
 12. The communications apparatus of claim 10, wherein apositive phase change represents a one bit and a negative phase changerepresents a zero bit.
 13. The communications apparatus of claim 1,wherein the at least one path bit P indicates a clockwise orcounter-clockwise rotation from a prior location of the symbol to acurrent location of the symbol.
 14. A communications method, comprising:generating a carrier waveform; modulating, using a modulator, thecarrier waveform according to a path-dependent phase modulation schemeto produce a modulated signal; and mapping, using a mapping module, areceived data payload to the path-dependent phase modulation scheme,each symbol S in the path-dependent phase modulation scheme including aconcatenation of at least one location bit L and at least one path bitP.
 15. The method of claim 14, wherein the at least one location bitcorresponds to a current location of the symbol in a symbolconstellation corresponding to the path-dependent phase modulationscheme, and the at least one path bit indicates a clockwise orcounter-clockwise rotation from a prior location of the symbol to thecurrent location of the symbol in a complex plane of the symbolconstellation.
 16. The method of claim 15, further comprising:controlling the modulator, based on an output from the mapping module,to impose the path-dependent phase modulation scheme on the carrierwaveform to generate the modulated signal.
 17. The method of claim 15,wherein the at least one location bit represents a value of lower orderbits at the current location of the symbol.
 18. The method of claim 14,wherein the at least one location bit indicates an amount of a phasetransition in the modulated signal and the at least one path bitindicates a direction of the phase transition.
 19. The method of claim14, wherein each symbol, S, is defined by S=[PL], where L corresponds tothe at least one location bit and represents log₂(2^(N)) bits, therebeing N symbol locations defined in a symbol constellation correspondingto the path-dependent phase modulation scheme, and P corresponds to theat least one path bit.
 20. The method of claim 14, wherein the at leastone path bit P indicates a clockwise or counter-clockwise rotation froma prior location of the symbol to a current location of the symbol.